Electromagnetic forming apparatus having improved backing member of high strength and electrical resistance



y 1967 M. A. FERGUSON 3,321,946

FORMING AP 1 Filed Dec. l 64 ELECTROMAGNETIC PARATUS HAVING IMPROVEDBACKING MEMBER OF HIGH STRENGTH AN LECTRICAL RESISTANCE #1? P/JC'I POWER SUPPLY ATTORNEY United States. Patent 3,321,946 ELECTROMAGNETIC FORMING APPARATUS HAVING IMPROVED BACKING MEMBER OF STRENGTH AND ELECTRICAL RESIST- Millard A. Ferguson, Utica, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Dec. 16, 1964, Ser. No. 418,792 6 Claims. (Cl. 72-56) This invention relates to electromagnetic forming apparatus, and particularly to improvements in an electromagnetic forming coil.

The electromagnetic forming process, because it develops very substantial magnetic pressures for forming purposes, demands that the forming coil be of sufiicient strength to withstand the resultant reaction pressures that act radially outwardly when an internally positioned workpiece is formed. Since generally those materials having the conductivity requirements do not meet the strength requirements, the coils must usually be increased in cross-sectional area and/ or backed with insulating material. However, the increased area renders the coil ineflicient since the currents traversing the external parts of the coil are loosely coupled to the workpiece and therefore contribute very little to its forming. Then too, the various kinds of insulating material used to back the coil lack strength and hence the insulator must be quite large. These large insulators, of course, increase the size of the over-all assembly. These problems become even more acute when it is desired to use a single turn coil.

To overcome these problems and others, a new and different electromagnetic forming coil is proposed that not only provides adequate backing for the conductor itself, but also inhibits the. straying of the current from the conductor thus providing a more eificient coil. Specifically, a backingimember of a high strength and of a high electric resistance, conductive materialis employed and constructed so as to incorporate a series of current flow inhibitors.

The foregoing and other objects and advantages of the invention will become apparent from the following description and accompanying drawings in which:

FIGURE 1 is a sectional view of an electromagnetic forming coil incorporating the principles of the invention; I

FIGURE 2 is a front elevational view of the coil lookin g in the direction of arrows 2-2 in FIGURE 1; and

FIGURE 3 is an exploded view of the coil.

Considering now the drawing in detail and referring first to FIGURE 1, the electromagnetic forming coil illustrated comprises a single turn conductor or winding formed of a highly conductive material such as copper or copper beryllium. The winding 10, as perhaps best viewed in FIGURE 3, is cylindrical in shape and has about the inside diameter thereof an insulating strip 12. This insulating strip 12 prevents the stray of current into a workspace 14 defined by the winding 10 and in which the workpiece 16 is positioned. The workpiece 16 observed in FIGURE 1 consists of a tubular section 18 and a hub 20 to which the end of the tubular section 18 is fit clamped by magnetic forming pressure. Other types of 3,321,946 Patented May 30, 1967 30, a spark gap 32 and a manually operable switch 34. The capacitor bank 30 can be replaced by any suitable pulse source, such as a motor-generator set while the function of the spark gap 32 can be accomplished by a thyr-atron or ignitron.

In operation, when the manually operable switch 34 is closed, current at the voltage of the power supply 28; e.g., 10 kilovolts, is supplied to the capacitor bank 30. The capacitor bank 30 will charge to some predetermined voltage, which is adequate to ionize the spark gap 32. When this occurs, a current pulse of a high amperage, for instance 100,000 amperes, is supplied to the winding 10 and by way of the leads 22 and 24. This high amperage current will develop a very substantial magnetomotive force that in turn will create a very high intensity magnetic field within the workspace 14. The resultant magnetic pressure will deform the end of the tubular section 18 and clamp it onto the end of the hub 20 as shown in FIGURE 1 to provide the completed assemblage.

As has been mentioned, the magnetic forces required to accomplish the forming are substantial and consequently the reactive force acting radially outwardly must be withstood by the coil. Otherwise, no forming would take place. It becomes impracticable to increase the size of the winding 10 beyond a certain point since the current in the external or peripheral part of the winding 10 is loosely coupled to the workpiece and thus contributes little to the forming. It is also not practical to use a higher strength material since the resultant increased electric resistance will consume power and again decrease the efliciency. Therefore, a support or backing member, denoted by the numeral 36, is provided. The backing member 36 surrounds the winding 10 as illustrated and is joined as by brazing at its inner diameter 38 to the periphery of the winding 10. The backing member 36 is made of a high strength material that presents a high electric resistance. Such a material includes the ferrous alloys, particularly those containing large amounts of chromium; e.g., stainless steel. Since these high strength materials are conductive, there will be a tendency for the current in the winding 10 to flow into the backing member 36 even though it does provide a high resistance. To further inhibit these stray currents, the backing member 36 is furnished with a series of outwardly opening spaced slots 40 along the periphery thereof. In between the slots 40 there are provided through holes 42. The number of slots 40'and the holes 42 will of course be determined by the strength requirements of the backing member 36 and the currents that are to beused. The slots 40 and holes 42 will increase the impedance of the backing member 36 and thus maintain the resistive and inductive external currents at a minimum.

As will now be appreciated, the novel coil allows a single turn winding 10 to be employed without concern for its strength while avoiding the usual inefliciencies due to a loose coupling of the current in the outer diameter thereof to the workspace. The backing member 36 not only presents a high resistance to stray currents being formed of a high resistance material, but also the slots 40 and the holes 42 function as current fiow inhibitors and thus further increase the impedance of the backing member 36 to the stray currents. This enables high strength materials to be employed to not only gain the rigidity requirements, but also to reduce the over-all size of the coil.

The invention is to be limited only by the following claims.

What is claimed is:

1. In apparatus for electromagnetically forming workpieces, the combination of a current conductor defining a workspace of a predetermined shape for the workpiece to be formed by magnetic pressures, a support member in electrical contact with the conductor, the support member being formed of a conductive material having a high structural strength for resisting reactive forces imposed upon the conductor by the magnetic pressures and a high electric resistance material and having parts thereof adapted to cause the support member to present a high impedance to current flow therein to combine with the high electric resistance to inhibit current stray from the conductor.

2. In apparatus for electromagnetically forming workpieces, the combination of a coil-shaped current conductor defining a workspace of a predetermined shape for the workpiece, a backing member surrounding the conductor and in electrical contact therewith, the backing member being formed of a conductive material having a high structural strength for resisting radial forces imposed upon the conductor during the magnetic forming and a high electric resistance and having current fioW impedances arranged therein to combine with the high electric resistance to inhibit current stray from the conductor.

3. In apparatus for electromagnetically forming workpieces, the combination of a current conductor defining a workspace of a predetermined shape :for the workpiece to be formed by magnetic pressures, a support member joined in electrical contact with the conductor for restraining movement of the conductor against reactive forces imposed thereon by the magnetic pressures, the support member being formed of a chromium ferrous alloy and having a series of current flow interrupters arranged so as to inhibit current stray from the coil.

4. In apparatus for electromagnetically forming workpieces, the combination of a coilashaped current conductor defining an internal workspace of a predetermined shape for the workpieces, a backing member joined to and surrounding the conductor, the backing member being formed of a conductive material having a high structural strength for resisting radial forces imposed upon the conductor during the magnetic forming and a high electric resistance and having a series of spaced openings arranged therein so as to with the high electric resistance present a high impedance to current flow in the backing member thereby inhibiting current stray from the conductor.

5. In electromagnetic forming apparatus, the combination of a current conductor of a single turn configuration defining a predetermined workspace, the conductor having an insulator joined to the inside thereof, a backing member joined to and surrounding the conductor so as to support the conductor against radially outwardly acting forces imposed thereon during the magnetic forming, the backing member being formed of a chromium ferrous alloy and having a series of spaced current flow inhibiting openings arranged therein so as to facilitate the prevention of current stray from the conductor.

6. In an electromagnetic forming apparatus for workpieces, the combination of a single turn coil for current, the coil being of a highly conductive material and having an insulator joined to the inside thereof to define a workspace of a predetermined shape for the workpiece, a backing member in mechanical and electrical contact with the coil and in an enclosing relationship therewith, the backing member being formed of a conductive material having a high structural strength for resisting radially outwardly acting forces imposed upon the coil during the magnetic forming and a high electric resistance and having a series of slots spaced about the periphery thereof and an opening between each of the slots so as to cause the backing member to present with the high electric resistance a high impedance to current flow therein thereby inhibiting stray current from the coil.

References Cited UNITED STATES PATENTS 2,976,907 3/1961 Harvey et al. 3,088,200 5/1963 Birdsall et al. 3,108,325 10/1963 Harvey et al. 3,183,126 5/1965 Curtis et al. 336-179 BERNARD A. GILHEANY, Primary Examiner.

H. A. LEWI'ITER, Assistant Examiner. 

2. IN APPARATUS FOR ELECTROMAGNETICALLY FORMING WORKPIECES, THE COMBINATION OF A COIL-SHAPED CURRENT CONDUCTOR DEFINING A WORKSPACE OF A PREDETERMINED SHAPE FOR THE WORKPIECE, A BACKING MEMBER SURROUNDING THE CONDUCTOR AND IN ELECTRICAL CONTACT THEREWITH, THE BACKING MEMBER BEING FORMED OF A CONDUCTIVE MATERIAL HAVING A HIGH STRUCTURAL STRENGTH FOR RESISTING RADIAL FORCES IMPOSED UPON THE CONDUCTOR DURING THE MAGNETIC FORMING AND A HIGH ELECTRICAL RESISTANCE AND HAVING CURRENT FLOW IMPEDANCES ARRANGED THEREIN TO COMBINE WITH THE HIGH ELECTRIC RESISTANCE TO INHIBIT CURRENT STAY FROM THE CONDUCTOR. 